This application relates to a novel method and means for effecting allotropic changes in various elements and compounds, and more particularly to the use of acoustical energy to effect the cavitation of a liquid metal that is seeded with graphite particles, thereby to convert the graphite to diamond.
An allotropic change is one in which the structure of a solid is transformed from one stable form to another without any chemical change. A specific allotropic transformation that this invention is designed to bring about is that of graphite to diamond. Other substances that can be induced to undergo useful allotropic changes by this invention are boron nitride (BN) and zinc sulphide (ZnS). For a given material, the structure that is being transformed will be called Form I, and the new structure will be called Form II. Thus carbon has graphite as Form I, and diamond as Form II.
The basic principal of this invention is best understood within the context of cavitation dynamics, a technical field that deals with the motions of cavitation bubbles generated in a "host" liquid by an acoustical pressure. The relevant concepts and nomenclature are set forth in two papers by the applicant (H. G. Flynn, J. Acoust. Soc. Am. Vol. 57, pp. 1379-1396, and J. Acoust. Soc. Vol. 58, pp. 1160-1170, each 1975) as well as in applicant's above-noted patent application Ser. No. 907,737. In this nomenclature the word cavity is used to distinguish cavitation bubbles from passive spheres of gas and vapor found, for example, in carbonated liquids.
When subjected to a reduction of pressure of an appropriate duration and magnitude, small pre-existing bubbles of gas and vapor in a liquid of the type described herein expand to some maximum size and then collapse with a great violence. An intense, shock wave is radiated by the collapsed cavity. This phenomenon is called cavitation, and when properly controlled it may result in an implosion of a bubble that causes very high energy densities both within the bubble and in the surrounding liquid. In the case of a specific embodiment of this invention, these high energy densities are used to convert graphite or carbon particles to diamond.
The transition from graphite to diamond by shock waves has been reported by De Carli and Jameson (Science Vol. 113, p. 1821, 1961) and by Adler and Christian (Physical Review Letters, Vol. 7, p. 367, 1961). Bundy (J. Chem. Phys. Vol. 38, p. 631, 1963) identified a region in the pressure-temperature phase diagram for carbon (referred to hereinafter as the S-T region) where essentially spontaneous transitions from graphite to diamond occur. Data on the graphite-diamond transition have been summarized by Bundy, Strong and Wentorf (The Chemistry and Physics of Carbon, Vol. 10, Marcel Dekker, Inc., 1973). Shock-induced transformation of graphite to diamond has also been described in U.S. Pat. Nos. 3,328,018; 3,238,019; 3,348,918; 3,401,019; 3,608,014; 3,632,242; 3,659,072.
None of these publications, however, suggests the conversion of elements or compounds from Form I to Form II structures by an acoustical cavitation process of the type described hereinafter.
In what follows, the symbol R.sub.n denotes the initial or equilibrium radius of a bubble in centimeters, and the symbol R denotes the time-varying radius of a bubble with an initial radius R.sub.n. Negative pressure means a reduction of the operating (static, ambient) pressure p.sub.L by an applied, time-dependent pressure field p.sub.A (known also as acoustic pressure), which may or may not make the total pressure less than zero. The operating (static, ambient) temperature is denoted by T.sub.L. The small bubbles of inert gas from which cavitation starts are called seeds; the liquid in which cavitation occurs is called the host liquid; and a method of obtaining a distribution of seeds is called seeding the liquid. As noted hereinafter, seeding can be effected by exposing graphite particles to an inert gas before the particles are placed in the host liquid.
The cycle of expansion and contraction that a bubble undergoes under the influence of the applied pressure field p.sub.A is called a cavitation event, the term being restricted to the growth of a bubble from a seed and its subsequent collapse. The region in the host liquid where these events take place is called the cavitation zone. During the negative part of the cycle, a seed in the cavitation zone may grow into a bubble whose maximum radius R.sub.o may be many times greater than the initial radius R.sub.n. The bubble then collapses to a minimum radius R.sub.m. The ratio R.sub.o /R.sub.n is called the expansion ratio and the ratio R.sub.m /R.sub.o is called the compression ratio. Bubbles that pulsate about their equilibrium radii R.sub.n with maximum expansion ratios R.sub.o /R.sub.n of less than 2 are called stable cavities, while bubbles that have expansion ratios much greater than 2, and which collapse extremely rapidly, are called transient cavities.
A transient cavity is an amplifier of energy density, in the sense that after expansion in a pressure field of low energy density the collapse of a transient cavity results in an implosion that generates very large energy densities both within the cavity and the surrounding liquid. The mechanism that enables a transient cavity to act as an energy density amplifier is the spherical convergence of the surrounding liquid during collapse At the end of collapse the energy stored in the liquid and in the cavity is converted into energy that drives an intense radiated shock wave. Thus during the implosion there is compression of the cavity contents and the surrounding liquid, and then the radiated shock wave causes the liquid to undergo a second compression. Behind this radiated shock front there will be a "shell" of compression that propagates outwardly. This shell is a region in the host liquid in which pressures and temperatures are high enough to bring about allotropic transformations of Form I within the shell.
The spherical convergence of the liquid surrounding an imploding cavity causes the cavity interface to become unstable in the sense that any small perturbation of the cavity interface will grow. Control of such initial perturbations by methods described hereinafter delays break up of the interface until the energy density has reached a maximum. Then, whether the interface remains spherical or not, the very large kinetic and internal energy densities generated by the imploding cavity will be available for driving the intense shock wave radiated by the collapsed cavity.
For a seed of given initial radius there is a negative time-dependent pressure of magnitude P.sub.t, called the pressure threshold for transient cavitation. For negative pressures with magnitudes greater than P.sub.t, seeds of that specified size will grow into transient cavities. This critical pressure increases with an increase in the operating (static, ambient) pressure and decreases with an increase in the initial radius of the seed and an increase in the duration of the negative pressure. For a seed of radius R.sub.n there is a negative timedependent pressure of magnitude P.sub.t called the pressure threshold for transient cavitation. For negative pressures with magnitudes greater than P.sub.t, seeds of that specified size will grow into transient cavities.